RC Circuits

Related Physics Education Research

There is a good deal of research on student understanding of DC circuits that bears on student understanding of RC circuits. See, for example, L. C. McDermott and P. S. Shaaffer, American Journal of Physics 60, 994-1003 (1992). For a more complete list, see the resource paper cited on my ILI homepage.

Overview of Worksheet

Students build on their understanding of circuits to build a conceptual and mathematical understanding of RC circuits. The students are asked to derive the differential equation of the circuit and its solution. They then turn to experiment to see the actual time behavior of current in an RC circuit.

Approximate Time

Two to three hours for the whole sheet, which includes consideration of both charging and discharging RC circuits.

Required Student Background

Students need to have a solid conceptual understanding of circuits and why current flows in a circuit. In order to do the whole worksheet, students need to have some familiarity with simple differential equations (linear with constant coefficients). However, if their math is not yet that sophisticated, they can still do a large portion of the sheet.

Connections to the Tutorials

This sheet builds on student conceptual understanding that is developed in the two A Model for circuits tutorials. While there is an RC circuit tutorial, it does not have as much calculus as this worksheet.

Required Equipment

A 1M Ohm Resistor and a micro Farad capacitor, power supply, buffer amp, dual channel amplifier and MBL equipment. The buffer amp increases the input impedence of the MBL equipment and allows us to use a large resistor and small capacitor. If you don't have a buffer amp, you can use about a smaller resistor and a very large capacitor (e.g., 20 Ohm resistor and 30,000 micro Farad capacitor). The results are not quite as good (because the capacitor is not perfect), but are qualitatively correct.

Evaluation of Worksheet

86% (32/37) students who used this worksheet correctly identified the time evolution of current in a discharging circuit (out of five choices) as compared to 67% (27/40) of students who did not use this worksheet but who did a similar experiment with RC circuits.